Drug delivery and lung deposition from a dry powder inhaler (DPI) are dependent on the peak inspiratory flow (PIF) through the DPI. Therefore, when prescribing a DPI, it is important to know whether a child is able to generate sufficient PIF through a particular device. Using a PIF meter (In-Check Dial) that mimics the internal resistance of DPIs, two commonly used devices (high-resistance Turbohaler (TH) and low-resistance Diskus (DK)) determined the PIF generated by asthmatic children through each of them. Two hundred and twenty-three children were studied, of whom 100 (mean age, 9.1 +/- 3.0 years; range, 3-15 years) were experienced with the use of a DPI (>1 month of regular DPI use), and 123 (mean, 5.5 +/- 1.9 years; range, 3-9 years) were inexperienced (no previous DPI use). All of the experienced patients generated more than 30 l/min through both devices, but a PIF of 60 l/min through the TH was obtained by only 68 (68%) of them. The age above which a minimal PIF of 30 l/min (for DK) or 60 l/min (for TH) could be achieved in new DPI users (inexperienced) was 4 years and 9 years, respectively. Even among experienced patients, many young children may not generate optimal PIFs through high-resistance DPIs. When DPI treatment is considered for young children, some devices may be successfully introduced at a younger age. It may thus be important to measure PIF in children who use a DPI or in whom DPI use is contemplated. This evaluation can be easily undertaken in the clinic with the In-Check Dial device.